As an indicator of diseases like cancer, DNA is a natural target for development of biosensors. Of particular biological and medical interest are sequences derived from infectious organisms or oncogenic cells. Dr. H. Holden Thorp's laboratory has demonstrated that femtomolar quantities of DNA sequences important in cancer research can be electrochemically detected through oxidation of guanine in DNA by the soluble inorganic complex, tris (2.2'-bipyridine)ruthenium(II) - [Ru(bpy)3]2+. The long-term objective of the research proposed here is to develop an electrochemical DNA-detection assay using a carrageenan hydrogel matrix to confine Ru(bpy)32+ and DNA so that neither species can migrate away from the electrode surface. Specific aim (i) is to electrochemically detect guanine oxidation by Ru(bpy)32+ in a carrageenan hydrogel. This specific aim requires incorporation of guanine into a carrageenan hydrogel containing Ru(bpy)32+, determination of the optimal electrode system, and investigation of the kinetics of guanine oxidation in carrageenan hydrogels. Specific aim (ii) is to develop an electrochemical DNA-detection system using a carrageenan hydrogel and Ru(bpy)32+. This specific aim requires attachment of oligonucleotide probes to the ruthenium-doped carrageenan hydrogel followed by quantification of guanine oxidation. The method for probe/target incorporation into the hydrogel will rely on covalent attachment of single-stranded probes or the use of hybridized biotinylated-probe/target complexes with avidin-labeled carrageenan.